In the automotive sector, systems which measure the distances and velocities of objects around one's own vehicle by using microwaves and applying the radar principle are in use. These objects can be vehicles which are actively taking part in the highway traffic or some sort of obstacles on or near the road. Keyless remote-entry systems for vehicles (keyless entry/comfort entry/keyless go systems) also make use of these technologies. In the known systems, high-frequency energy is radiated in a frequency range in the gigahertz range, at a mid-frequency of 24,125 GHz and with a two-way bandwidth of several GHz. Typical antennas have a directional characteristic (i.e., an antenna radiation pattern) of 80 degrees*20 degrees. In practice, the transmission range is about 20 m. The risk inherent in such systems is that unacceptably high signal levels occur, even in frequency ranges that have been blocked in favor of other services, e.g., frequency ranges that are reserved for radio astronomy or also for radio relay services. Unacceptably high signal levels can occur, for example, when a substantial number of the above-mentioned systems in the surrounding, for example several hundred, are simultaneously put into operation. This can be the case, for example, when a large number of vehicles are moving on multilane urban streets. Similar problems arise in large parking lots at sports facilities or shopping centers when, for example, after a big event ends, hundreds of vehicles start moving at the same time and leave the parking lot. For the most part, these problems only occur when the vehicles are at standstill or traveling at a relatively slow speed. This is because, at higher speeds, the distances between the vehicles increase again, and the vehicle density decreases correspondingly. Furthermore, the spatial proximity of many sensors also causes heavy mutual interference, which, when working with adaptive sensors, increasingly leads to additional measurements being taken, although some objects may have actually already been reliably detected.
Published German Patent Application DE 100 65 521 describes a method and a device for detecting moving or stationary objects using radar radiation, in particular for use in motor vehicles, where, in order to detect objects, pulse-modulated carrier waves are radiated, whose reflected portions are then received and evaluated. In this context, by transmitting an unmodulated carrier in the time intervals between two adjacent pulses, a Doppler measurement can additionally be performed, thereby enabling a reliable velocity measurement to be taken.
When irregularities are detected in received signals, the transmitting branch of the radar may be switched off. Thus, no more transmission signals are emitted by the transmitting antenna. However, correlation pulses from a pulse transmitter continue to be transmitted to the receiving branch of the radar sensor. If it turns out in the process that object information is still received, then an illusory object must be inferred.